1. Field of the Invention
The present invention relates to a dielectric ceramic composition which is widely used in high frequency electronic components and, more particularly, to a low-temperature cofired dielectric ceramic composition with a high dielectric constant and a low dielectric loss.
Low-temperature cofired dielectric compositions (materials) means compositions(materials) which can be fired in the temperature range of 800-950xc2x0 C., lower than the melting point of silver (Ag) or copper (Cu), in contrast to the conventional ceramic dielectrics which are sintered at 1,300xc2x0 C. or higher.
2. Description of the Prior Art
The strong recent tendency toward miniaturization, lightness and intelligence of electric and electronic appliances has demanded a considerable reduction in the size and number of substituent devices on circuit boards as well as the high performance thereof. To meet the demands, there have been made extensive attempts in some of which part-on-boards are multi-layered, followed by firing dielectrics and electrodes simultaneously.
However, high sintering temperatures of conventional dielectric materials require the use of high melting temperature metals, such as Mo or W, in inner electrode patterns in order to achieve the multilayering and simultaneous firing of circuit boards and device parts.
Where Mo or W is adopted in inner electrode patterns, however, an economic disadvantage is incurred because of its high cost. Above all, the skin effect, which causes radio frequency current to stay near the surface of a conductor, requires that metals of low electric resistance be used for the electrode patterns in order to reduce the dielectric loss. Accordingly, there have been indispensably used metals that are relatively inexpensive as well as being of high electric conductivity, like Ag or Cu.
As Ag or Cu is used in inner electrode patterns, a very important research subject is to find a dielectric material which can be fired at lower than the melting point of Ag (960xc2x0 C.) or Cu (1083xc2x0 C.).
Typically, low-temperature fired dielectrics are prepared by liquid-phase sintering an admixture comprising high-temperature fired material with a high dielectric constant and low dielectric loss in combination with a small amount of a low melting point material, for example, glass powder or an additive such as CuO, PbO and Bi2O3, V2O5, etc., or by firing glass ceramics comprising ceramics as a filler.
When the latter is used, the resulting dielectric substrates based on glass show a dielectric constant of 10 or less.
With advantages of low dielectric constants in speeding-up signal processing and improving signal transmission, materials with a low dielectric constant of 10 or less are extensively used for low temperature cofired ceramics (LTCC).
Meanwhile, depending on characteristics of applied circuits, substrates made of dielectrics with low dielectric loss and medium dielectric constant (15-100) may often enjoy advantages in terms of circuit design and function without retardation of signal processing.
As a rule, however, use of dielectrics with high dielectric constants makes the guided wavelength short, leading to a reduction in circuit dimension. Thus, such dielectrics are very useful in applications which attach importance to dimensions of electric elements, as well as having the advantage of reducing insertion loss properties or frequency deviation according to circuits.
Larger dielectric constants can lower the ratio of the width of transmission lines to the thickness of dielectrics to greater extents, giving circuit designers an opportunity to design better lamination structures.
ZrO2xe2x80x94AOxe2x80x94B2O5xe2x80x94TiO2 (A=Zn, Mg, Co, Mn, and B=Nb, Ta)-based dielectric compositions with dielectric constants of approximately 40 or higher are disclosed in U.S. Pat. No. 5,470,808. These dielectric compositions can be fired at temperatures of 1,300xc2x0 C. or higher, which are too high to co-fire Ag electrodes. That is, ZrO2xe2x80x94AOxe2x80x94B2O5xe2x80x94TiO2 (A=Zn, Mg, Co, Mn, and B=Nb, Ta)-based dielectric compositions alone have difficulty in being used for TLCC.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and provide a dielectric ceramic composition which exhibits high dielectric constant and low dielectric loss and can be sintered at low temperature.
It is another object of the present invention to provide a dielectric ceramic composition which is improved in sintering properties as well as being controllable in high frequency dielectric properties.
In accordance with an aspect of the present invention, there is provided a dielectric ceramic composition represented by the following chemical formula 1:
xe2x80x83a wt. % {xZrO2xe2x88x92yZnOxe2x88x92wNb2O5xe2x88x92zTiO2}+cwt. % Glass Fritxe2x80x83xe2x80x83Chemical Formula 1
wherein, 5.0 mol %xe2x89xa6xxe2x89xa645.0 mol %; 1.5 mol % xe2x89xa6yxe2x89xa619.0 mol %; 1.5 mol %xe2x89xa6wxe2x89xa619.0 mol %; 40.0 mol %xe2x89xa6zxe2x89xa659.0 mol % with the proviso that x+y+w+z=100; 75.0xe2x89xa6axe2x89xa697.0; and 3.0xe2x89xa6cxe2x89xa625.0.
In accordance with another aspect of the present invention, there is provided a dielectric ceramic composition represented by the following chemical formula 2:
a wt. % {xZrO2xe2x88x92yZnOxe2x88x92wNb2O5xe2x88x92zTiO2}+bwt. % (MgO, CoO, SiO2, Sb2O3, Sb2O5, MnO2, Ta2O5 or combinations thereof)+cwt. % ZnOxe2x80x94B2O3xe2x80x94SiO2 based Glass Fritxe2x80x83xe2x80x83Chemical Formula 2
wherein, 5.0 mol % xe2x89xa6xxe2x89xa645.0 mol %; 1.5 mol %xe2x89xa6yxe2x89xa619.0 mol %; 1.5 mol %xe2x89xa6wxe2x89xa619.0 mol %; 40.0 mol %xe2x89xa6zxe2x89xa659.0 mol % with the proviso that x+y+w+z=100; 75.0xe2x89xa6axe2x89xa697.0; bxe2x89xa61.5; and 3.0xe2x89xa6cxe2x89xa625.0).
In accordance with a further aspect of the present invention, there is provided a dielectric ceramic composition represented by the following chemical formula 3:
a wt. % {xZrO2xe2x88x92yZnOxe2x88x92wNb2O5xe2x88x92zTiO2}+cwt. % ZnOxe2x80x94B2O3xe2x80x94SiO2 based Glass Frit+dwt. %CuOxe2x80x83xe2x80x83Chemical Formula 3
wherein, 5.0 mol %xe2x89xa6xxe2x89xa645.0 mol %; 1.5 mol %xe2x89xa6yxe2x89xa619.0 mol %; 1.5 mol %xe2x89xa6wxe2x89xa619.0 mol %; 40.0 mol %xe2x89xa6zxe2x89xa659.0 mol % with the proviso that x+y+w+z=100; 75.0xe2x89xa6axe2x89xa697.0; 3.0xe2x89xa6cxe2x89xa625.0; and dxe2x89xa65.0.
In accordance with still a further aspect of the present invention, there is provided a dielectric ceramic composition represented by the following chemical formula 4:
a wt. % {xZrO2xe2x88x92yZnOxe2x88x92wNb2O5xe2x88x92zTiO2}+bwt. % (MgO, CoO, SiO2, Sb2O3, Sb2O5, MnO2, Ta2O5 or combinations thereof)+cwt. % ZnOxe2x80x94B2O3xe2x80x94SiO2 based Glass Fritxe2x80x83xe2x80x83Chemical Formula 4
wherein, 5.0 mol %xe2x89xa6xxe2x89xa645.0 mol %; 1.5 mol %xe2x89xa6yxe2x89xa619.0 mol %; 1.5 mol %xe2x89xa6wxe2x89xa619.0 mol %; 40.0 mol %xe2x89xa6zxe2x89xa659.0 mol % with the proviso that x+y+w+z=100; 75.0xe2x89xa6axe2x89xa697.0; bxe2x89xa61.5; 3.0xe2x89xa6cxe2x89xa625.0; and dxe2x89xa65.0)
Based on ZrO2xe2x80x94ZnOxe2x80x94Nb2O5xe2x80x94TiO2 with low dielectric loss and high dielectric constant ( greater than 45), the dielectric composition of the present invention comprises ZnOxe2x80x94B2O3xe2x80x94SiO2 glass frit as a sintering aid, thereby exhibiting a high electric constant of 30 or more and low dielectric loss (Q greater than 1,000 (at 3 GHz), Q≈1/tan xcex4), and being able to be cofired with Ag electrode patterns.
To the composition, at least one oxide selected from the group consisting of MgO, CoO, SiO2, Sb2O3, Sb2O5, MnO2, and Ta2O5, and/or CuO may be further incorporated. In the dielectric composition, the oxide is used to improve dielectric properties while CuO acts as a sintering aid.
As described above, the ceramic composition of ZrO2xe2x80x94ZnOxe2x80x94Nb2O5xe2x80x94TiO2 is low in dielectric loss and 45 or higher in dielectric constant and is sintered at 1,300xc2x0 C. The high sintering temperature makes it impossible to sinter the ceramic composition with electrodes made of Ag whose melting point is 961xc2x0 C.
In accordance with the present invention, the base ceramic composition ZrO2xe2x80x94ZnOxe2x80x94Nb2O5xe2x80x94TiO2 is modified in the molar ratio of its constituting ingredients, and is incorporated with a certain amount of glass frit so as to make it possible to co-fire the ceramic composition with the Ag electrode. For use in the present invention, the base ceramic composition ZrO2xe2x80x94ZnOxe2x80x94Nb2O5xe2x80x94TiO2 comprises ZrO2 (x) in an amount of 5.0xcx9c45 mol %, ZnO (y) in an amount of 1.5xcx9c19.0 mol %, Nb2O5 (W) in an amount of 1.5xcx9c19.0 mol %, and TiO2 (z) in an amount of 40xcx9c59.0 mol % with the proviso that x+y+w+z=100.
With ZnO or Nb2O5 in an amount less than 1.5 mol %, the base ceramic composition is not sintered at 1,300xc2x0 C. such that its dielectric properties cannot be measured. More than 1.5 mol %, both ZnO and Nb2O5 can function to improve sintering properties. Increasing of ZnO and Nb2O5 contents causes the base ceramic composition to increase in dielectric constant as well as in temperature coefficient of frequency (TCF) from the negative to the positive direction. On the other hand, at more than 19.0 mol % of ZnO or Nb2O5, the sintering density decreased while the temperature coefficient of resonant frequency is excessively increased in the positive direction.
Below 5 mol % of ZrO2, the temperature coefficient of resonant frequency is too high in the positive direction to apply the base ceramic composition in practice. On the other hand, when the content of ZrO2 is over 45 mol %, the base ceramic composition is not sintered even at 1,400xc2x0 C.
As for TiO2, its content is defined in the range of 40-59.0 mol % by the predetermined molar ratios of ZrO2, ZnO and Nb2O5.
Useful in the present invention is a ZnOxe2x80x94B2O3xe2x80x94SiO2 based glass frit. Preferably, it comprises ZnO in an amount of 30-70 wt %, B2O3 in an amount of 5-30 wt %, SiO2 in an amount of 5-40 wt %, and PbO in an amount of 2-40 wt %.
B2O3 lowers the viscosity of the glass and accelerates the densification of the dielectric ceramic composition of the present invention. Where B2O3 is used in an amount lower than 5 wt. %, the dielectric ceramic composition is likely to not be sintered at lower than 900xc2x0 C. With more than 30 wt % of B2O3, the dielectric ceramic composition has poor moisture resistance. Thus, its amount is preferably in the range of 5-30 wt. % in the glass frit.
More than 40 wt % of SiO2 results in an excessive increase in the softening temperature of the glass frit which therefore cannot act as a sintering aid. When SiO2 is present in an amount less than 5 wt %, its effect is not obtained. That is, a preferable amount of SiO2 falls within the range of 5-40 wt. %.
With less than 2 wt % of PbO, the glass frit has too high a softening temperature (Ts), making no contribution to the densification of the dielectric ceramic composition. On the other hand, more than 40 wt. % of PbO lowers the Ts of the glass frit to improve the densification of the composition, but has the problem of decreasing Q value. Considering these facts, the amount of PbO in the glass frit is defined in the range of 2-40 wt %.
It is preferred that ZnO is used in an amount of 30-70 wt %. Excessive amounts of ZnO lead to an increase in the softening temperature of the glass frit, making the low temperature firing impossible.
In accordance with another embodiment of the present invention, at least one oxide selected from the group consisting of MgO, CoO, SiO2, Sb2O3, Sb2O5, MnO2, and Ta2O5 is further used in the dielectric ceramic composition of the present invention to improve dielectric properties. With similarity to the main components ZrO2, ZnO, Nb2O5 and TiO2 in electric charge and ionic radius, the oxides MgO, CoO, SiO2, Sb2O3, Sb2O5, MnO2, and Ta2O5 affect the ionic bonds of the main components to decrease the dielectric loss, functioning to increase the Q value without a large change of dielectric constant and the temperature coefficient of resonant frequency. The oxide is used in an amount of 1.5 wt % or less. At more than 1.5 wt % of the additive oxide, a drastic decrease is brought about in both dielectric constant and Q value.
In accordance with a further embodiment of the present invention, CuO is further used in the dielectric ceramic composition of the present invention. In cooperation with the glass frit, CuO serves as a sintering aid to increase the dielectric constant. Also, CuO plays a role in controlling the temperature coefficient of frequency without a large change in Q value. It is preferably used in an amount of 5 wt % or less. More than solubility limit in the dielectric, CuO exists in the interface, leading to a drastic increase in Q value.
Below, a description will be given of the preparation of the dielectric ceramic composition of the present invention.
The starting materials ZrO2, ZnO, Nb2O5, and TiO2 with a purity of 99.0% or higher, are weighed according to a desired composition of x ZrO2xe2x88x92y ZnOxe2x88x92w Nb2O5xe2x88x92z TiO2, and admixed in a wet manner. In this regard, the wet mixing is carried out by milling the starting materials in deionized water for about 16 hours with the aid of 3"PHgr" zirconia balls in a rod mill. The slurry thus obtained is dried and calcined. Preferably, the calcination is carried out at 1,000-1,030xc2x0 C. for about 2 hours at the heating rate of 5xc2x0 C./min. When the calcination temperature is lower than 1,000xc2x0 C., much ZrO2 remains in an unreacted phase, giving rise to an increase in shrinkage. At higher than 1,030xc2x0 C., on the other hand, the powder becomes too coarse to pulverize later.
After being weighed according to a desired composition, the glass frit components are melted at 1,200-1,400xc2x0 C., quenched in water, and dry-pulverized. Then, the coarse particles are finely pulverized into powder with a size of 0.5xcx9c1.0 xcexcm in ethyl alcohol.
The base dielectric ceramic composition is admixed with the glass frit powder composition, together with appropriate amounts of CuO and at least one additive selected from the group consisting of MgO, CoO, SiO2, Sb2O3, Sb2O5, MnO2, and Ta2O5, in a batch, and the admixture is pulverized.
Following drying, the powder thus obtained was secondarily calcined at 600-700xc2x0 C. The secondary calcination temperature, which is somewhat higher than the softening temperature (Ts) of the glass frit, makes the dielectric homogenous with the glass frit, thereby improving the uniformity of the dielectric ceramic composition after the sintering.
Next, the calcined powder is further broken down into a desired particle size, mixed with a binder, and molded to a desired form such as a disc or a sheet.
Afterwards, the electrode in a form of disc or sheet is calcined and co-fired at less than 900xc2x0 C. to produce a desired device.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.